# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Message:αβγ
Message:abc
 Add Images & Files Posting Code: Name: Email:
Re: The development of bubble sextants
From: Frank Reed
Date: 2009 Aug 15, 08:26 -0700

```Quoting Gary:
"Coriolis is a fictitious force use to explain the perceived curve of the
flight path as observed by an observer on earth of other rotating frame
of reference."

Coriolis acceleration is a "frame-dependent" acceleration. That is, it depends
on the choice of coordinates that you use to define motion. The expression
"fictitious force" is a technical term intended to describe this
frame-dependence. Just so there's no misunderstanding, it DOES NOT mean that
the Coriolis acceleration is "fictional" in the ordinary sense of the English
word. Of course, Coriolis acceleration is "real" in the sense that in many
circumstances you find that the natural coordinates to describe the problem
are rotating coordinates. The weather on Earth or any other planet, for that
matter, is nearly impossible to understand in non-rotating coordinates but
quite easy to understand in coordinates which rotate.

From a modern perspective (modern = since the early part of the 20th century,
after the development of general relativity), even the common acceleration of
gravity near the surface of the Earth (at a single point) at 9.8 m/s^2 or 32
ft/sec^2 is a "fictitious force" since it can be eliminated by going to a
frame of reference which is accelerating toward the Earth's center, in other
words a freefall frame. And sure enough, if you place an aircraft on a
parabolic trajectory with that acceleration, gravity disappears and
passengers are rendered completely weightless exactly as if they are in orbit
(until the plane's acceleration trajectory is changed). It is not "as if"
there is no gravity; in that frame of reference, there really is "no gravity"
apart from local tidal accelerations.

Many people are able to explain the origin of the Coriolis acceleration by
describing how it appears in an inertial, non-rotating frame of reference. In
such a frame, the object moves on a straight line "while the Earth turns
beneath it". This is very important information, of course, but it is a
derivation, like an "etymology". It tells us why this acceleration must exist
in non-inertial frames of reference, but it doesn't mean that it is fake or
"fictional". And you surely wouldn't want to revert to the derivation once
you understand why it works.

In many ways, the expression "fictitious force" in physics has caused as many
problems as the expression "imaginary number" in mathematics. Neither of
these concepts are invalid or fictional or "un-real". For modern mathematics,
so-called imaginary numbers have no defects, no lack of reality to them. They
are what they are: solutions to algebraic equations. Likewise in physics,
so-called fictitious forces are not forces to be avoided or treated as
"un-real". They are what they are: forces arising from the choice of
coordinates.

-FER

--~--~---------~--~----~------------~-------~--~----~
NavList message boards: www.fer3.com/arc
Or post by email to: NavList@fer3.com
To unsubscribe, email NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~---
```
Browse Files

Drop Files

### Join NavList

 Name: (please, no nicknames or handles) Email:
 Do you want to receive all group messages by email? Yes No
You can also join by posting. Your first on-topic post automatically makes you a member.

### Posting Code

Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
 Email:

### Email Settings

 Posting Code:

### Custom Index

 Subject: Author: Start date: (yyyymm dd) End date: (yyyymm dd)